Lubricating oil composition and method for using lubricating oil composition

ABSTRACT

A lubricating oil composition including a base oil (X) having a tertiary carbon atom at a content of 8.0 at % or more based on the total carbon of hydrocarbons and an antioxidant (Y) having an amine-based antioxidant (A), a phenol-based antioxidant (B), and a phosphor-based antioxidant (C). The amine-based antioxidant (A) includes a diphenylamine compound (A1) represented by the following general formula (a1): 
     
       
         
         
             
             
         
       
     
     where R a1  and R a2 , na1 and na2 are defined in the disclosure.

TECHNICAL FIELD

The present invention relates to a lubricating oil composition, and amethod for using the lubricating oil composition.

BACKGROUND ART

Lubricating oil compositions used in devices such as turbines such assteam turbines and gas turbines, compressors, and hydraulic equipmentare used while circulating in a system under a high-temperatureenvironment. Therefore, oxidation deterioration easily occurs. Undersuch circumstances, for said lubricating oil compositions, variousresearches and developments to finely maintain oxidation stabilityagainst long period of time use under high temperature environments arebeing conducted.

For example, PTL 1 discloses a lubricating oil composition for a rotarygas compressor, which contains a lubricant base oil having a viscosityindex of 120 or more, phenyl-α-naphthylamine or a derivative thereof,p,p′-dialkyldiphenylamine or a derivative thereof, and a viscosity indeximprover.

According to PTL 1, the lubricating oil composition can be a lubricatingoil composition for a rotary gas compressor that achieves both thermaland oxidation stability and sludge resistance at a high level even whenused at a high temperature, and at the same time has an excellent energysaving effect.

CITATION LIST Patent Literature

-   PTL 1: JP 2011-162629 A

SUMMARY OF INVENTION Technical Problem

Based on diversification of raw material procurement, etc., the presentinventors did intensive studies on lubricating oil compositions that areused for devices such as turbines, gas compressors and hydraulicdevices, using various base oils as base oils. As a result, they foundthat, in the case where a base oil containing tertiary carbon atom at acertain amount or more is used as a base oil, oxidation stability cannotbe maintained for a long period of time by blending an antioxidant toconventionally-used base oils such as synthetic oils such as polyα-olefins and polyalkylene glycols (PAGs) and mineral oils.

On the other hand, as an index for indicating the oxidation stability oflubricating oil compositions, an RPVOT value (Rotating Pressure VesselOxidation Test) defined in JIS K 2514-3:2013 or ASTM D 2272 is mainlyused, and it is general to evaluate the residual lifetime of an oil usedfrom the decrease in performance of an antioxidant. Evaluation using anRPVOT value (RBOT value) is carried out also in PTL 1.

However, under the environment for use in an actual device, an oil comesout of the system accompanying with compressed air ejected when thelubricating oil composition is used by being circulated in a device, andthus the device is used while suitably filling the lubricating oilcomposition so that a predetermined amount of lubricating oilcomposition is maintained in the actual device in many cases. Therefore,there was a problem that oxidation stability was not be able to beevaluated properly by only the results of the evaluation of RPVOT valuescarried out under a tightly-closed condition. Incidentally, when theraising velocity of an acid value is fast, an RPVOT value also tends tobe short; therefore, suppression of raising in an acid value at theinitial stage consequently leads to maintenance of oxidation stabilityfor a long period of time.

Therefore, from the viewpoint to evaluate oxidation stability properlyfor a base oil containing a certain amount or more of tertiary carbonatom, for which it was impossible to maintain oxidation stability for along period of time by blending an antioxidant in a base oil which hasbeen conventionally used, it is desired not only to make the oxidationstability evaluated by an RPVOT value excellent but also to suppressraising of an acid value (an acid value raising velocity) at the initialstage so that the oxidation stability is maintained for a long period oftime.

SUMMARY OF INVENTION Technical Problem

Therefore, the object of the present invention is to provide alubricating oil composition capable of maintaining oxidation stabilityover a long period of time by not only making oxidation stabilityevaluated by an RPVOT value excellent even while using a base oilcontaining a certain amount or more of tertiary carbon atom but alsosuppressing a rise in acid value in the initial stage (acid valueraising velocity).

Solution to Problem

The present inventors did intensive studies and consequently found thatthe above-mentioned problem can be solved by a combination of specificantioxidants and adjustment of the total content of these specificantioxidants, and completed the present invention.

Specifically, the present invention relates to the following [1] and[2].

[1] A lubricating oil composition comprising:

a base oil (X) comprising tertiary carbon atom at a content of 8.0 at %or more based on the total carbon of hydrocarbons; and

an antioxidant (Y) comprising an amine-based antioxidant (A), aphenol-based antioxidant (B), and a phosphor-based antioxidant (C),

wherein the amine-based antioxidant (A) comprises a diphenylaminecompound (A1) represented by the following general formula (a1):

wherein R^(a1) and R^(a2) are each independently an alkyl group having 1to 30 carbon atoms, na1 and na2 are each independently an integer of 1to 5, wherein when na1 is 2 or more, a plurality of R^(a1)'s may be thesame, or may be different from each other, and in the case where na2 is2 or more, a plurality of R^(a2)'s may be the same, or may be differentfrom each other,

the phenol-based antioxidant (B) comprises a hindered phenol compound(B1) having an ester structure, and

the total content [(A1)+(B1)+(C)] of the diphenylamine compound (A1),the hindered phenol compound (B1) having an ester structure, and thephosphor-based antioxidant (C) based on the total amount of thelubricating oil composition is greater than 1.0 mass % and 2.0 mass % orless.

[2] A method for using the lubricating oil composition according to [1]in one or more device selected from the group consisting of a turbine, acompressor and a hydraulic device.

Advantageous Effects of Invention

The present invention can provide a lubricating oil composition capableof maintaining oxidation stability over a long period of time by notonly making oxidation stability evaluated by an RPVOT value excellenteven while using a base oil containing a certain amount or more oftertiary carbon atom but also suppressing a rise in acid value in theinitial stage.

DESCRIPTION OF EMBODIMENTS

In this specification, regarding a preferable numerical range (forexample, a range of contents), the lower limit value and upper limitvalue which are described stepwise can be each independently combined.For example, from the description “preferably 10 to 90, more preferably30 to 60”, “preferable lower limit value (10)” and “more preferableupper limit value (60)” can be combined to form “10 to 60”.

Furthermore, in this specification, the numerical values in Examples arenumerical values that can be used as the upper limit value or lowerlimit value.

In this specification, unless otherwise stated, a numerical rangeexpressed as “AA to BB” means “AA or more and BB or less”.

[Embodiment of Lubricating Oil Composition of Present Invention]

The lubricating oil composition of the present invention is alubricating oil composition containing: a base oil (X) containingtertiary carbon atom at a content of 8.0 at % or more based on the totalcarbon of hydrocarbons; and an antioxidant (Y) containing an amine-basedantioxidant (A), a phenol-based antioxidant (B), and a phosphor-basedantioxidant (C).

The amine-based antioxidant (A) contains a diphenylamine compound (A1)represented by the following general formula (a1).

In the formula, R^(a1) and R^(a2) are each independently an alkyl grouphaving 1 to 30 carbon atoms, na1 and na2 are each independently aninteger of 1 to 5, wherein when na1 is 2 or more, a plurality ofR^(a1)'s may be the same, or may be different from each other, and inthe case where na2 is 2 or more, a plurality of R^(a2)'s may be thesame, or may be different from each other.

The phenol-based antioxidant (B) contains a hindered phenol compound(B1) having an ester structure.

Furthermore, the total content [(A1)+(B1)+(C)] of the diphenylaminecompound (A1), the hindered phenol compound (B1) having an esterstructure, and the phosphor-based antioxidant (C) based on the totalamount of the lubricating oil composition is greater than 1.0 mass % and2.0 mass % or less.

The present inventors did intensive studies aiming at providing alubricating oil composition capable of not only making oxidationstability evaluated by an RPVOT value excellent but also maintainingoxidation stability over a long period of time by suppressing rising ofan acid value in the initial stage even while using a base oilcontaining a certain amount or more of tertiary carbon atom.

First, the inventors investigated that the reason why an antioxidant ispoorly effective in blending an antioxidant in conventionally-used baseoils such as synthetic oils such as poly α-olefin and PAG and mineraloils, in the case where a base oil containing a certain amount or moreof tertiary carbon atom is used. As a result, it was considered that oneof the causes is that, since radicals of tertiary carbon atom are morestable as compared to lower carbon radicals than tertiary carbon atom,there are more radicals that can capture oxygen atoms in a base oilcontaining a certain amount or more of tertiary carbon atom, and thusoxidation deterioration occurs easily. Furthermore, from this fact, itis considered that, in the case where a base oil containing a certainamount or more of tertiary carbon atom is used, blending of anantioxidant is more difficult to adjust than before. Actually, when anantioxidant was blended in a similar manner to conventional ones in abase oil containing a certain amount or more of tertiary carbon atom,the antioxidant was poorly effective, and thus it was very difficult toprepare a lubricating oil composition that solves the above-mentionedproblem.

Under such circumstances, the present inventors did intensive studies onblend of an antioxidant, and found the blend of an antioxidant which cansolve the above-mentioned problem, and completed the present invention.

In the following explanation, “base oil (X)” and “antioxidant (Y)” arealso referred to “component (X)” and “component (Y)”, respectively.

Although the lubricating oil composition of one aspect of the presentinvention may be constituted from only the component (X) and thecomponent (Y), it may contain other components other than the component(X) and the component (Y) in the scope in which the effect of thepresent invention is not deteriorated.

In the lubricating oil composition of one aspect of the presentinvention, the total content of the component (X) and the component (Y)is preferably 80 mass % or more, more preferably 85 mass % or more,further preferably 90 mass % or more, further more preferably 95 mass %or more, even further preferably 97 mass % or more, further morepreferably 99.9 mass % or less, based on the total amount of thelubricating oil composition.

Hereinafter the respective components contained in the lubricating oilcomposition of the present invention will be explained in detail.

<Base Oil (X)>

The lubricating oil composition of the present invention contains a baseoil (X).

The base oil (X) contains tertiary carbon atom at a content of 8.0 at %or more based on the total carbons of hydrocarbons in the base oil (X).

According to the studies of the present inventors, it has been foundthat, in the case where the tertiary carbon atom content in the base oil(X) is 8.0 at % or more based on the total carbons of hydrocarbons inthe base oil (X, oxidation deterioration of the base oil (X) occurseasily, and it is more difficult to adjust blend of an antioxidant thanbefore. In the present invention, by adjusting the blend of anantioxidant, which will be mentioned later, raising of an acid value issuppressed while maintaining oxidation stability for a long period oftime.

Incidentally, in one aspect of the present invention, the tertiarycarbon atom content in the base oil (X) may be 8.5 at % or more, may be9.0 at % or more, may be 9.5 at % or more, or may be 10.0 at % or more.Incidentally, the upper limit value of the tertiary carbon atom contentin the base oil (X) is generally 20.0 at % or less.

Incidentally, the tertiary carbon atom content based on the totalcarbons of hydrocarbons in the base oil (X) means a rate of the total ofthe integrated intensity attributable to the tertiary carbon atom withrespect to the total of the all integrated intensities of carbonmeasured by ¹³C-NMR, but other method may also be used as long as asimilar result can be obtained. Incidentally, in ¹³C-NMR measurement, amethod using a sample obtained by adding 3 g of deuterated chloroform to0.5 g of a sample to be measured and diluting the sample, in which themeasurement temperature is room temperature, the resonance frequency is100 MHz, and the measurement method is a method using a gated decouplingmethod, may be exemplified. By the above-mentioned analysis, (a) and (b)are measured respectively as mentioned below, and the ratio of (b) (%)when (a) is set as 100% is calculated. The ratio of (b) represents theratio of the total tertiary carbon atom atom against the total carbonatom constituting the base oil (X).

(a) Total of the integrated intensities at chemical shifts of about10-50 ppm (total of the integrated intensities attributable to the totalcarbon of hydrocarbons)(b) Total of the integrated intensities at chemical shifts of about27.9-28.1 ppm, 28.4-28.6 ppm, 32.6-33.2 ppm, 34.4-34.6 ppm, 37.4-37.6ppm, 38.8-39.1 ppm and 40.4-40.6 ppm (the total of the integratedintensities attributable to tertiary carbon atoms and naphthene tertiarycarbon atoms with methyl groups, ethyl groups, and other branchedgroups)

Incidentally, in the case where the base oil (X) is a mixed base oil, avalue obtained by measuring the tertiary carbon atom content of saidmixed base oil by the above-mentioned method is indicated.

(Content of Base Oil (X)

In the lubricating oil composition of one aspect of the presentinvention, the content of the base oil (X) is preferably 90.0 mass % ormore, more preferably 93.0 mass % or more, further preferably 95.0 mass% or more based on the total amount of the lubricating oil composition.Furthermore, the content is preferably lower than 99.0 mass %, morepreferably 98.5 mass % or less, further preferably 98.0 mass % or less.

(Kinds of Base Oil (X))

As the base oil (X), a base oil containing tertiary carbon atom at acontent of 8.0 at % or more based on the total carbons of hydrocarbonsin the base oil (X) can be used without any specific limitation.Hereinafter a “base oil containing tertiary carbon atom at a content of8.0 at % or more based on the total carbons of hydrocarbons in the baseoil (X)” is also referred to as a “high tertiary carbon atom-containingbase oil”.

Typical examples of the high tertiary carbon atom-containing base oilinclude, for example, ordinary pressure residual oils, which areobtained by ordinary pressure distillation of crude oils such asparaffin-based mineral oils, medium-based mineral oils andnaphthene-based mineral oils; distilled oils obtained by reducedpressure distillation of said ordinary pressure residual oils; mineraloils or waxes (slack wax, GTL wax, etc.) obtained by subjecting saiddistilled oils to one or more treatment from purification treatmentssuch as solvent deasphalting, solvent extraction, hydrogenationfinishing, solvent dewaxing, contact dewaxing, isomerization dewaxingand reduced pressure distillation and isoparaffin polymers.

These base oils (X) may be formed of one kind, or may be a mixed oilincluding two or more kinds in combination.

Incidentally, the base oil (X) may contain one kind or two or more kindsof base oils containing tertiary carbon atom at a content of lower than8.0 at % such as synthetic oils and plant oils, within a range in whichthe content of tertiary carbon atom satisfies 8.0 at % or more based onthe total amount of the base oil (X).

However, from the viewpoint of making the effect of the presentinvention to exert easier, the base oil (X) is contained by preferably50 mass % to 100 mass % more preferably 60 mass % to 100 mass %, furtherpreferably 70 mass % to 100 mass %, further more preferably 80 mass % to100 mass %, and further even preferably 90 mass % to 100 mass %, basedon the total amount of the base oil.

(Kinematic Viscosity and Viscosity Index of Base Oil (X) at 40° C.)

The kinematic viscosity of the base oil (X) at 40° C. (hereinafter alsoreferred to as “40° C. kinematic viscosity”) is preferably 10 mm²/s to100 mm²/s, more preferably 30 mm²/s to 60 mm²/s, further preferably 35mm²/s to 55 mm²/s, further more preferably 40 mm²/s to 50 mm²/s.

Furthermore, the viscosity index of the base oil (X) is preferably 100or more, more preferably 125 or more, further preferably 130 or more.Furthermore, the viscosity index is generally 150 or less.

In this specification, the 40° C. kinematic viscosity and viscosityindex of the base oil (X) are values measured and calculated accordingto JIS K 2283:2000.

<Antioxidant (Y)>

The lubricating oil composition of the present invention contains anantioxidant (Y).

In the case where the lubricating oil composition does not contain theantioxidant (Y), the effect of the present invention is not exerted.

The antioxidant (Y) contained by the lubricating oil composition of thepresent invention contains an amine-based antioxidant (A), aphenol-based antioxidant (B), and a phosphor-based antioxidant (C). Inthe following explanations, these are respectively referred to as“component (A)”, “component (B)” and “component (C)”.

In the lubricating oil composition of one aspect of the presentinvention, the antioxidant (Y) may be constituted by only the component(A), the component (B) and the component (C), but may contain otherantioxidants than the component (A), the component (B) and the component(C) in the scope in which the effect of the present invention is notdeteriorated.

In the lubricating oil composition of one aspect of the presentinvention, the total content of the component (A), the component (B) andthe component (C) is preferably 60 mass % to 100 mass %, more preferably70 mass % to 100 mass %, further preferably 80 mass % to 100 mass %,further more preferably 90 mass % to 100 mass %, even further preferably95 mass % to 100 mass % based on the total amount of the antioxidant(Y), from the viewpoint of making the effect of the present inventionexert more easily.

Hereinafter, the amine-based antioxidant (A), the phenol-basedantioxidant (B) and the phosphor-based antioxidant (C) are explained indetail.

<Amine-Based Antioxidant (A)>

The lubricating oil composition of the present invention contains anamine-based antioxidant (A) as the antioxidant (Y).

The amine-based antioxidant (A) is a compound having an anti-oxidationperformance and containing an amino group.

Hereinafter the amine-based antioxidant (A) used in the lubricating oilcomposition of the present invention is explained in detail.

(Diphenylamine Compound (A1))

In the lubricating oil composition of the present invention, theamine-based antioxidant (A) contains a diphenylamine compound (A1)represented by the following general formula (a1).

In the general formula (a1), R^(a1) and R^(a2) are each independently analkyl group having 1 to 30 carbon atoms, na1 and na2 are eachindependently an integer of 1 to 5, wherein when na1 is 2 or more, aplurality of R^(a1)'s may be the same, or may be different from eachother, and in the case where na2 is 2 or more, a plurality of R^(a2)'smay be the same, or may be different from each other.

According to the study of the present inventors, the diphenylaminecompound (A1) has a performance to make oxidation stability evaluated byan RPVOT value fine by using with the phosphor-based antioxidant (C) incombination. On the other hand, the diphenylamine compound (A1) tends toraise an acid value at the initial stage. Therefore, in the presentinvention, in order to maintain oxidation stability for a long period oftime by suppressing raising of oxidation at the initial stage, the totalcontent [(A1)+(B1)+(C)] of the diphenylamine compound (A1), the hinderedphenol compound (B1) having an ester structure and the phosphor-basedantioxidant (C) is defined to be within a predetermined range.Incidentally, in the case where the lubricating oil composition does notcontain the diphenylamine compound (A1), the effect of the presentinvention is not exerted.

In the general formula (a1), the carbon number of the alkyl group thatcan be selected as R^(a1) and R^(a2) is each independently preferably 1to 20, more preferably 4 to 16, further preferably 4 to 14 from theviewpoint of making the effect of the present invention exert moreeasily.

Specific examples of the alkyl group that can be selected as R^(a1) andR^(a2) include a methyl group, an ethyl group, a propyl group, a butylgroup, a pentyl group, a hexyl group, a heptyl group, an octyl group, anonyl group, a decyl group, an undecyl group, a dodecyl group, atridecyl group, a tetradecyl group, a pentadecyl group, a hexadecylgroup, a heptadecyl group, an octadecyl group, a nonadecyl group, anicosyl group, a henicosyl group, a dococyl group, a tricocyl group, atetracocyl group, a pentacocyl group, a hexacocyl group, a heptacocylgroup, an octacocyl group, a nonacocyl group and a triacontyl group.

These alkyl groups may be linear or branched, and are preferably linearfrom the viewpoint of making the effect of the present invention exertmore easily.

In the above-mentioned general formula (a1), na1 and na2 are eachindependently preferably 1 to 3, more preferably 1 to 2, furtherpreferably 1, from the viewpoint of making the effect of the presentinvention exert more easily.

For the diphenylamine compound (A1), one kind may be used alone, or twoor more kinds may be used in combination.

The content of the diphenylamine compound (A1) is preferably 0.30 mass %to 1.8 mass %, more preferably 0.30 mass % to 1.2 mass %, furtherpreferably 0.40 mass % to 1.1 mass % based on the total amount of thelubricating oil composition.

Since the content of the diphenylamine compound (A1) is in theabove-mentioned range, raising of an acid value at the initial stage ofthe lubricating oil composition is easily suppressed while makingoxidation stability of the lubricating oil composition evaluated by anRPVOT value fine.

(Naphthylamine Compound (A2))

In the lubricating oil composition of one aspect of the presentinvention, from the viewpoint of making the effect of the presentinvention exert more easily, it is preferable that the amine-basedantioxidant (A) contains a naphthylamine compound (A2) represented bythe following general formula (a2).

In the general formula (a2), R^(a3) is an alkyl group having 1 to 30carbon atoms, na3 is an integer of 1 to 5, and when na3 is 2 or more, aplurality of R^(a3)′ may be the same or may be different from eachother.

In the general formula (a2), the carbon number of the alkyl group thatcan be selected as R^(a3) is preferably 1 or more, more preferably 4 ormore, further preferably 6 or more from the viewpoint of making theeffect of the present invention exert more easily. Furthermore, thecarbon number is preferably 20 or less, more preferably 16 or less,further preferably 14 or less. The upper limit value and lower limitvalue of these numerical ranges can be arbitrarily combined.Specifically, the combination is preferably 1 to 20, more preferably 4to 16, further preferably 6 to 14.

Specific examples of the alkyl group that can be selected as R^(a3) mayinclude those exemplified as the alkyl groups that can be selected asR^(a1) and R^(a2).

These alkyl groups may be linear or branched, and are preferably linearfrom the viewpoint of making the effect of the present invention exertmore easily.

In the above-mentioned general formula (a2), na3 is preferably 1 to 3,more preferably 1 to 2, further preferably 1 from the viewpoint ofmaking the effect of the present invention exert more easily.

For the naphthylamine compound (A2), one kind can be used alone, or twoor more kinds can be used in combination.

The content of the naphthylamine compound (A2) is preferably 0.50 mass %or more, more preferably 0.55 mass % or more, further preferably 0.60mass % or more based on the total amount of the lubricating oilcomposition, from the viewpoint of making the effect of the presentinvention exert more easily. Furthermore, the content is preferably 0.80mass % or less, more preferably 0.75 mass % or less, further preferably0.70 mass % or less. The upper limit value and lower limit value ofthese numerical ranges can be arbitrarily combined. Specifically, thecombination is preferably 0.50 mass % to 0.80 mass %, more preferably0.55 mass % to 0.75 mass %, further preferably 0.60 mass % to 0.70 mass%.

(Contents and Content Ratio of Various Amine Compounds in Amine-BasedAntioxidant (A))

In the lubricating oil composition of one aspect of the presentinvention, the content of the diphenylamine compound (A1) is preferably30 mass % to 100 mass %, more preferably, 35 mass % to 100 mass %,further preferably 40 mass % to 100 mass % based on the total amount ofthe amine-based antioxidant (A), from the viewpoint of making the effectof the present invention exert more easily.

Furthermore, in the lubricating oil composition of one aspect of thepresent invention, in the case where the amine-based antioxidant (A)contains the naphthylamine compound (A2), the total content of thediphenylamine compound (A1) and the naphthylamine compound (A2) ispreferably 80 mass % to 100 mass %, more preferably 90 mass % to 100mass %, further preferably 95 mass % to 100 mass %, further morepreferably 98 mass % to 100 mass %, even further preferably 99 mass % to100 mass % based on the total amount of the amine-based antioxidant (A),from the viewpoint of making the effect of the present invention exertmore easily.

Incidentally, in the lubricating oil composition of one aspect of thepresent invention, the content of the other amine-based antioxidant (A)other than the diphenylamine compound (A1) and the naphthylaminecompound (A2) is preferably small from the viewpoint of making theeffect of the present invention exert more easily. The content of saidother amine-based antioxidant (A) is preferably lower than 5 mass %,more preferably lower than 1 mass %, further preferably lower than 0.1mass %, further more preferably lower than 0.01 mass % based on thetotal amount of the amine-based antioxidant (A), even further preferablyfree from said other amine-based antioxidant (A′).

Said other amine-based antioxidant (A) may include, for example,aralkylated diphenylamines, which do not fall within the diphenylaminecompound (A1).

(Total Content and Content Ratio of Diphenylamine Compound (A1) andNaphthylamine Compound (A2))

In the lubricating oil composition of one aspect of the presentinvention, in the case where the amine-based antioxidant (A) containsthe naphthylamine compound (A2), the total content [(A1)+(A2)] of thediphenylamine compound (A1) and the naphthylamine compound (A2) ispreferably 0.80 mass % or more, more preferably 0.90 mass % or more,further preferably 1.0 mass % or more based on the total amount of thelubricating oil composition, from the viewpoint of making the effect ofthe present invention exert more easily. Furthermore, the total contentis preferably lower than 2.0 mass %, more preferably 1.9 mass % or less,further preferably 1.7 mass % or less. The upper limit value and lowerlimit value of these numerical ranges can be arbitrarily combined.Specifically, the combination is preferably 0.80 mass % or more andlower than 2.0 mass %, more preferably 0.90 mass % to 1.9 mass %,further preferably 1.0 mass % to 1.7 mass %.

Furthermore, in the lubricating oil composition of one aspect of thepresent invention, in the case where the amine-based antioxidant (A)contains the naphthylamine compound (A2), the content ratio [(A1)/(A2)]of the diphenylamine compound (A1) and the naphthylamine compound (A2)is preferably 0.5 or more, more preferably 0.6 or more, furtherpreferably 0.7 or more by mass ratio, from the viewpoint of making theeffect of the present invention exert more easily. Furthermore, thecontent ratio is preferably 2.0 or less, more preferably 1.8 or less,further preferably 1.7 or less. The upper limit value and lower limitvalue of these numerical ranges can be arbitrarily combined.Specifically, the combination is preferably 0.5 to 2.0, more preferably0.6 to 1.8, further preferably 0.7 to 1.7.

<Phenol-Based Antioxidant (B)>

The lubricating oil composition of the present invention contains thephenol-based antioxidant (B) as the antioxidant (Y).

The phenol-based antioxidant (B) is a compound having an anti-oxidationperformance and having a phenol structure. However, of such compounds,compounds containing a phosphor atom are deemed to be encompassed in thephosphor-based antioxidant (C).

Hereinafter the phenol-based antioxidant (B) used in the lubricating oilcomposition of the present invention will be explained in detail.

(Hindered Phenol Compound (B1) Having Ester Structure)

In the lubricating oil composition of the present invention, thephenol-based antioxidant (B) contains a hindered phenol compound (B1)having an ester structure.

According to the study by the present inventors, the hindered phenolcompound (B1) having an ester structure has a performance to suppressoxidation deterioration at the initial stage, which is easy to occur ina high tertiary carbon atom-containing base oil. However, the esterstructure moiety of said hindered phenol compound (B1) is hydrolyzedwith a minute amount of moisture contained in the high tertiary carbonatom-containing base oil, and easily raises an acid value at the initialstage. Therefore, in the present invention, the total content[(A1)+(B1)+(C)] of the diphenylamine compound (A1), the hindered phenolcompound having an ester structure (B1) and the phosphor-basedantioxidant (C) is defined to be within a predetermined range.

In the case where the lubricating oil composition does not contain thehindered phenol compound (B1) having an ester structure, the effect ofthe present invention is not exerted.

Incidentally, the hindered phenol compound is a compound in which ahydrogen atom on a phenol aromatic ring has been substituted with abulky substituent such as a tert-butyl group.

The hindered phenol compound (B1) having an ester structure includes,for example, one or more kind selected from the group consisting of acompound (B1-1) represented by the following general formula (b1-1) anda compound (B1-2) represented by the following general formula (b1-2).Incidentally, in the following explanation, the hindered phenol compound(B1) having an ester structure is also simply referred to as “hinderedphenol compound (B1)”.

In the formula (b1-1), R^(b1) is an alkylene group having 1 to 5 carbonatoms, and R^(b2) is an alkyl group having 1 to 25 carbon atoms.

In the formula (b1-2), R^(b3) and R^(b4) are each independently analkylene group having 1 to 5 carbon atoms, and m is an integer of 2 to4.

In the general formula (b1-1), the carbon number of the alkylene groupthat can be selected as R^(b1) is preferably 1 to 4, more preferably 1to 3, further preferably 1 to 2, from the viewpoint of making the effectof the present invention exert more easily.

Specific examples of the alkylene group that can be selected as R^(b1)include linear alkylene groups such as a methylene group, an ethylenegroup, an n-propylene group, an n-butylene group and an n-pentylenegroup; and branched alkylene groups such as an isopropylene group, anisobutylene group, a sec-butylene, a tert-butylene group, anisopentylene group and a neopentylene group.

In the general formula (b1-1), the carbon number of the alkyl group thatcan be selected as R^(b2) is preferably 2 or more, more preferably 4 ormore, further preferably 6 or more, from the viewpoint of making theeffect of the present invention exert more easily. Furthermore, thecarbon number is preferably 20 or less, more preferably 15 or less,further preferably 10 or less. The upper limit value and lower limitvalue of these numerical ranges can be arbitrarily combined.Specifically, the combination is preferably 2 to 20, more preferably 4to 15, further preferably 6 to 10.

Specific examples of the alkyl group that can be selected as R^(b2) mayinclude a methyl group, an ethyl group, a propyl group, a butyl group, apentyl group, a hexyl group, a heptyl group, an octyl group, a nonylgroup, a decyl group, an undecyl group, a dodecyl group, a tridecylgroup, a tetradecyl group, a pentadecyl group, a hexadecyl group, aheptadecyl group, an octadecyl group, a nonadecyl group, an icosylgroup, a henicosyl group, a dococyl group, a tricocyl group, atetracocyl group and a pentacocyl group. These may be linear orbranched.

When the compound (B1-1) represented by the general formula (b1-1) isexemplified, benzenepropanoic acid3,5-bis(1,1-dimethylethyl)-4-hydroxyalkyl esters, etc. may be included.

For the compound (B1-1) represented by the general formula (b1-1), onekind may be used alone, or two or more kinds may be used in combination.

In the general formula (b1-2), the carbon number of the alkylene groupthat can be selected as R^(b3) and R^(b4) is preferably 1 to 4, morepreferably 1 to 3, further preferably 1 to 2, from the viewpoint ofmaking the effect of the present invention exert more easily.

Specific examples of the alkylene group that can be selected as R^(b3)and R^(b4) include groups that are similar to the groups exemplified asR^(b2).

When the compound (B1-2) represented by the general formula (b1-2) isexemplified, pentaerythritoltetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], etc. may beincluded.

For the compound (B1-2) represented by the general formula (b1-2), onekind may be used alone, or two or more kinds may be used in combination.

For the hindered phenol compound (B1), one kind may be used alone, ortwo or more kinds may be used in combination.

The content of the hindered phenol compound (B1) is preferably 0.50 mass% or more, more preferably 0.55 mass % or more based on the total amountof the lubricating oil composition. Furthermore, the content ispreferably 0.80 mass % or less, more preferably 0.70 mass % or less,further preferably 0.65 mass % or less. The upper limit value and lowerlimit value of these numerical ranges can be arbitrarily combined.Specifically, the combination is preferably 0.50 mass % to 0.80 mass %,more preferably 0.55 mass % to 0.70 mass %, further preferably 0.55 mass% to 0.65 mass %.

Since the content of the hindered phenol compound (B1) is in theabove-mentioned range, raising of an acid value of the lubricating oilcomposition at the initial stage can be suppressed, and raising of anacid value caused by a hydrolysate generated by a reaction with minutemoisture that can be contained in a high tertiary carbon atom-containingbase oil can be suppressed.

(Content of Hindered Phenol Compound (B1) in Phenol-Based Antioxidant(B))

In the lubricating oil composition of one aspect of the presentinvention, the content of the hindered phenol compound (B1) ispreferably 80 mass % to 100 mass %, more preferably 90 mass % to 100mass %, further preferably 95 mass % to 100 mass %, further morepreferably 98 mass % to 100 mass %, even further preferably 99 mass % to100 mass % based on the total amount of the phenol-based antioxidant(B), from the viewpoint of making the effect of the present inventionexert more easily.

Incidentally, in the lubricating oil composition of one aspect of thepresent invention, it is preferable that the content of the otherphenol-based antioxidant (B′) than the hindered phenol compound (B1) issmall, from the viewpoint of making the effect of the present inventionexert more easily. The content of said other phenol-based antioxidant(B′) is preferably lower than 5 mass %, more preferably lower than 1mass %, further preferably lower than 0.1 mass %, further morepreferably lower than 0.01 mass %, even further preferably free fromsaid other phenol-based antioxidant (B′), based on the total amount ofthe phenol-based antioxidant (B).

Said other phenol-based antioxidant (B′) may include, for example,2,6-di-tert-butyl-p-cresol (DBPC), which does not fall within thehindered phenol compound (B1).

<Phosphor-Based Antioxidant (C)>

The lubricating oil composition of the present invention contains aphosphor-based antioxidant (C) as the antioxidant (Y.

The phosphor-based antioxidant (C) is a compound containing a phosphoratom, and has a performance to make oxidation stability evaluated by anRPVOT value fine and to maintain oxidation stability for a long periodof time, by using with the amine-based antioxidant (A) in combination.

In the case where the lubricating oil composition does not contain thephosphor-based antioxidant (C), oxidation stability evaluated by anRPVOT value cannot be made excellent, and the oxidation stability cannotbe maintained for a long period of time.

Hereinafter, the phosphor-based antioxidant (C), which is used in thelubricating oil composition of the present invention, will be explainedin detail.

(Phosphor Atom-Containing Compound (C1) Having Phenol Structure)

In the lubricating oil composition of one aspect of the presentinvention, it is preferable that the phosphor-based antioxidant (C)contains a phosphor atom-containing compound (C1) having a phenolstructure represented by the following general formula (c1).

In the general formula (c1), R^(c1), R^(c2), R^(c3) and R^(c4) are eachindependently a hydrogen atom or an alkyl group having 1 to 30 carbonatoms.

The alkyl groups that can be selected as R^(c1), R^(c2), R^(c3) andR^(c4) may include those similar to the alkyl groups that can beselected as the above-mentioned R^(a1) and R^(a2).

However, the carbon numbers of the alkyl groups that can be selected asR^(c1), R^(c2), R^(c3) and R^(c4) are each independently preferably 1 to20, more preferably 1 to 10, further preferably 1 to 6.

Here, it is preferable that the phosphor atom-containing compound (C1)having a phenol structure has a hindered phenol backbone from theviewpoint of making the effect of the present invention exert moreeasily. Therefore, the alkyl group that can be selected as R^(c1) andR^(c2) is preferably a branched alkyl group, more preferably a branchedalkyl group having 1 to 6 carbon atoms, and further preferably atert-butyl group.

The content of the phosphor atom-containing compound (C1) having aphenol structure is preferably 0.05 mass % or more, more preferably 0.07mass % or more, further preferably 0.09 mass % or more based on thetotal amount of the lubricating oil composition. Furthermore, thecontent is preferably 0.30 mass % or less, more preferably 0.20 mass %or less, further preferably 0.15 mass % or less. The upper limit valueand lower limit value of these numerical ranges can be arbitrarilycombined. Specifically, the combination is preferably 0.05 mass % to0.30 mass %, more preferably 0.07 mass % to 0.20 mass %, furtherpreferably 0.09 mass % to 0.15 mass %.

Since the content of the phosphor atom-containing compound (C1) having aphenol structure is within the above-mentioned range, oxidationstability can be maintained stably for a long period of time whilesuppressing the contents of the diphenylamine (A1) and the hinderedphenol compound (B1).

(Content of Phosphor Atom-Containing Compound (C1) Having PhenolStructure in Phosphor-Based Antioxidant (C))

In the lubricating oil composition of one aspect of the presentinvention, the content of the phosphor atom-containing compound (C1)having a phenol structure is preferably 80 mass % to 100 mass %, morepreferably 90 mass % to 100 mass %, further preferably 95 mass % to 100mass %, further more preferably 98 mass % to 100 mass %, even furtherpreferably 99 mass % to 100 mass %, based on the total amount of thephosphor-based antioxidant (C), from the viewpoint of making the effectof the present invention exert more easily.

Incidentally, in the lubricating oil composition of one aspect of thepresent invention, it is preferable that the content of the otherphosphor-based antioxidant (C′) than the phosphor atom-containingcompound (C1) having a phenol structure is small, from the viewpoint ofmaking the effect of the present invention exert more easily. Thecontent of said other phosphor-based antioxidant (C′) is preferablylower than 5 mass %, more preferably lower than 1 mass %, furtherpreferably lower than 0.1 mass %, further more preferably lower than0.01 mass %, even further preferably free from said other phosphor-basedantioxidant (C′), based on the total amount of the phosphor-basedantioxidant (C).

Said other phosphor-based antioxidant (C′) may include, for example,metal-based antioxidants containing a phosphor atom which do not fallwithin the phosphor atom-containing compound (C1) having a phenolstructure such as zinc dialkyldithiophosphates.

<Other Antioxidant (Y′)>

In the lubricating oil composition of one aspect of the presentinvention, it is preferable that the content of the other antioxidant(Y′) than the amine-based antioxidant (A), the phenol-based antioxidant(B) and the phosphor-based antioxidant (C) is small from the viewpointof making the effect of the present invention exert more easily. Thecontent of said other antioxidant (Y′) is preferably lower than 5 mass%, more preferably lower than 1 mass %, further preferably lower than0.1 mass %, further more preferably lower than 0.01 mass %, even furtherpreferably free from said other antioxidant (Y′), based on the totalamount of the antioxidant (Y).

The other antioxidant (Y′), may include, for example, sulfur-basedantioxidants and metal-based antioxidants.

<Total Content and Content Ratio of Respective Antioxidants>

Hereinafter, regarding the diphenylamine compound (A1), the hinderedphenol compound (B1) having an ester structure, and the phosphor-basedantioxidant (C), the total content and the content ratio are explainedin detail.

([(A1)+(B1)+(C)])

In the lubricating oil composition of the present invention, the totalcontent [(A1)+(B1)+(C)] of the diphenylamine compound (A1), the hinderedphenol compound (B1) having an ester structure and the phosphor-basedantioxidant (C) needs to be greater than 1.0 mass % and 2.0 mass % orless.

If [(A1)+(B1)+(C)] is 1.0 mass % or less, oxidation stability evaluatedby an RPVOT value cannot be made excellent, and oxidation stabilitycannot be maintained for a long period of time.

Furthermore, if [(A1)+(B1)+(C)] is more than 2.0 mass %, a lubricatingoil composition in which an acid value easily rises at the initial stageis formed.

Here, [(A1)+(B1)+(C)] is preferably 1.1 mass % or more, more preferably1.2 mass % or more, from the viewpoint of making the effect of thepresent invention exert more easily. Furthermore, [(A1)+(B1)+(C)] ispreferably 1.9 mass % or less, more preferably 1.8 mass % or less,further preferably 1.7 mass % or less. The upper limit value and lowerlimit value of these numerical ranges can be arbitrarily combined.Specifically, 1.1 mass % to 1.9 mass % are preferable, 1.1 mass % to 1.8mass % are more preferable, and 1.2 mass % to 1.7 mass % are furtherpreferable.

([(A1)+(B1)])

In the lubricating oil composition of one aspect of the presentinvention, the total content [(A1)+(B1)] of the diphenylamine compound(A1) and the hindered phenol compound (B1) having an ester structure ispreferably 0.9 mass % or more, more preferably 1.0 mass % or more,further preferably 1.1 mass % or more, from the viewpoint of making theeffect of the present invention exert more easily. Furthermore, thetotal content is preferably 1.9 mass % or less, more preferably 1.8 mass% or less, further preferably 1.7 mass % or less, further morepreferably 1.6 mass % or less. The upper limit value and lower limitvalue of these numerical ranges can be arbitrarily combined.Specifically, the combination is preferably 0.9 mass % to 1.9 mass %,more preferably 1.0 mass % to 1.8 mass %, further preferably 1.1 mass %to 1.7 mass %, further more preferably 1.1 mass % to 1.6 mass %.

([(A1)/(B1)])

In the lubricating oil composition of one aspect of the presentinvention, the content ratio [(A1)/(B1)] of the diphenylamine compound(A1) to the hindered phenol compound (B1) having an ester structure ispreferably 0.5 or more, more preferably 0.6 or more, further preferably0.7 or more, further more preferably 0.8 or more by mass ratio, from theviewpoint of making the effect of the present invention exert moreeasily. Furthermore, the content ratio is preferably 2.0 or less, morepreferably 1.9 or less, further preferably 1.8 or less, further morepreferably 1.7 or less. The upper limit value and lower limit value ofthese numerical ranges can be arbitrarily combined. Specifically, thecombination is preferably 0.5 to 2.0, more preferably 0.6 to 1.9,further preferably 0.7 to 1.8, further more preferably 0.8 to 1.7.

([(A1)/(C)])

In the lubricating oil composition of one aspect of the presentinvention, the content ratio [(A1)/(C)] of the diphenylamine compound(A1) to the phosphor-based antioxidant (C) is preferably 3.0 or more,more preferably 3.5 or more, further preferably 4.0 or more, furthermore preferably 4.5 or more, even further preferably 5.0 or more by massratio, from the viewpoint of making the effect of the present inventionexert more easily. Furthermore, the content ratio is preferably 15.0 orless, more preferably 13.0 or less, further preferably 12.0 or less,further more preferably 11.0 or less, even further preferably 10.0 orless. The upper limit value and lower limit value of these numericalranges can be arbitrarily combined. Specifically, the combination ispreferably 3.0 to 15.0, more preferably 3.5 to 13.0, further preferably4.0 to 12.0, further more preferably 4.5 to 11.0, even furtherpreferably 5.0 to 10.0.

([(B1)/(C)])

In the lubricating oil composition of one aspect of the presentinvention, the content ratio [(B1)/(C)] of the hindered phenol compound(B1) having an ester structure to the phosphor-based antioxidant (C) ispreferably 4.0 or more, more preferably 4.5 or more, further preferably5.0 or more, further more preferably 5.5 or more by mass ratio, from theviewpoint of making the effect of the present invention exert moreeasily. Furthermore, the content ratio is preferably 10.0 or less, morepreferably 9.0 or less, further preferably 8.0 or less, further morepreferably 7.0 or less, even further preferably 6.5 or less. The upperlimit value and lower limit value of these numerical ranges can bearbitrarily combined. Specifically, the combination is preferably 4.0 to10.0, more preferably 4.5 to 9.0, further preferably 5.0 to 8.0, furthermore preferably 5.5 to 7.0, even further preferably 5.5 to 6.5.

<Additives for Lubricating Oil>

The lubricating oil composition according to one aspect of the presentinvention may contain additives for a lubricating oil other than theantioxidant (Y) as long as the effects of the present invention are notimpaired.

Examples of the additives for a lubricating oil include extreme pressureagents, detergent dispersants, pour point depressants, viscosity indeximprovers, rust inhibitors, metal deactivators, anti-foaming agents, andfriction modifiers.

These additives for lubricating oil may be used alone or in combinationof two or more kinds thereof.

In this specification, additives such as viscosity index improvers andanti-foaming agents may be blended with other components in the form ofa solution dissolved in a diluent oil with consideration for handlingproperty and solubility in a mineral base oil (A). In such case, in thisspecification, the content of additives such as anti-foaming agents andviscosity index improvers is a content in terms of active ingredients(resin content equivalent) excluding the diluent oil.

The details of the above-mentioned additives for a lubricating oil willbe explained below.

(Extreme Pressure Agent)

Examples of the extreme pressure agent include phosphorus-based extremepressure agents such as phosphate esters, phosphite esters, acidicphosphate esters, acidic phosphite esters and dithiophosphoric acid;sulfur-phosphorus-based extreme pressure agents such asmonothiophosphate esters and dithiophosphate esters; halogen-basedextreme pressure agents such as chlorinated hydrocarbons; andorganometallic extreme pressure agents.

These extreme pressure agents may be used alone or in combination of twoor more kinds thereof.

When the lubricating oil composition according to one aspect of thepresent invention contains an extreme pressure agent, the content of theextreme pressure agent is preferably 0.01 to 10% by mass, morepreferably 0.03 to 5% by mass, and further more preferably 0.05 to 1.0%by mass, based on the total amount of the lubricating oil composition.

(Detergent Dispersant)

Examples of the detergent dispersant include metal sulfonates, metalsalicylates, metal phenates, organic phosphite esters, organic phosphateesters, organic phosphate metal salts, succinimide, benzylamine,succinate esters, and polyhydric alcohol esters.

Examples of the metals constituting the metal salt such as metalsulfonates are preferably alkali metals or alkaline earth metals, morepreferably sodium, calcium, magnesium, or barium, and further morepreferably calcium. The succinimide, benzylamine, and succinate estermay be boron-modified forms. When the lubricating oil compositionaccording to one aspect of the present invention contains a detergentdispersant, the content of the detergent dispersant is preferably 0.01to 10% by mass, more preferably 0.02 to 7% by mass, and further morepreferably 0.03 to 5% by mass, based on the total amount of thelubricating oil composition.

(Pour Point Depressant)

Examples of the pour point depressant may include polymethacrylateshaving a weight average molecular weight of about 50,000 to 150,000. Inthe case where the lubricating oil composition of one aspect of thepresent invention contains a pour point depressant, the content of thepour point depressant is preferably 0.01 to 5 mass %, more preferably0.02 to 2 mass %, based on the total amount of said lubricating oilcomposition.

(Viscosity Index Improver)

Examples of the viscosity index improver include polymers such asnon-dispersant-type polymethacrylates, dispersant-typepolymethacrylates, olefin-based copolymers (for example, anethylene-propylene copolymer), dispersant-type olefin-based copolymers,and styrene-based copolymers (for example, a styrene-diene copolymer, astyrene-isoprene copolymer).

When the lubricating oil composition according to one aspect of thepresent invention contains a viscosity index improver, the content ofthe viscosity index improver in terms of a resin content is preferably0.01 to 10% by mass, more preferably 0.02 to 7% by mass, and furthermore preferably 0.03 to 5% by mass, based on the total amount of thelubricating oil composition.

(Rust Inhibitor)

Examples of the rust inhibitor include metal sulfonates,alkylbenzenesulfonates, dinonylnaphthalenesulfonates, organic phosphiteesters, organic phosphate esters, organic sulfonic acid metal salts,organic phosphoric acid metal salts, alkenyl succinic acid esters, andalkenyl succinic acid polyhydric alcohol esters.

When the lubricating oil composition according to one aspect of thepresent invention contains a rust inhibitor, the content of the rustinhibitor is preferably 0.01 to 10.0% by mass, and more preferably 0.03to 5.0% by mass, based on the total amount of the lubricating oilcomposition.

(Metal Deactivator)

Examples of the metal deactivator include benzotriazole compounds,tolyltriazole compounds, thiadiazole compounds, imidazole compounds, andpyrimidine compounds.

When the lubricating oil composition according to one aspect of thepresent invention contains a metal deactivator, the content of the metaldeactivator is preferably 0.01 to 5.0% by mass, and more preferably 0.03to 3.0% by mass, based on the total mass of the lubricating oilcomposition.

(Anti-Foaming Agent)

Examples of the anti-foaming agent include silicone-based anti-foamingagents, fluorine-based anti-foaming agents such as fluorosilicone oilsand fluoroalkyl ethers, and polyacrylate-based anti-foaming agents.

When the lubricating oil composition according to one aspect of thepresent invention contains an anti-foaming agent, the content of theanti-foaming agent in terms of a resin content is preferably 0.0001 to0.20% by mass, and more preferably 0.0005 to 0.10% by mass, based on thetotal mass of the lubricating oil composition.

(Friction Modifier)

Examples of the friction modifier include molybdenum-based frictionmodifiers such as molybdenum dithiocarbamate (MoDTC) and molybdenumdithiophosphate (MoDTP); and ash-free friction modifiers having at leastone alkyl or alkenyl group having 6 to 30 carbon atoms in the molecule,such as aliphatic amines, fatty acid esters, fatty acids, aliphaticalcohols, and aliphatic ethers.

Incidentally, it is preferable that the content of the friction modifiercontaining a sulfur atom, such as MoDTC or MoDTP, is small from theviewpoint of suppressing the precipitation of sludge generated with longperiod of time use under a high-temperature environment. Specifically,the content of the sulfur atom-containing friction modifier ispreferably lower than 1 mass %, more preferably lower than 0.5 mass %,further preferably lower than 0.1 mass %, further more preferably lowerthan 0.05 mass %, even further preferably lower than 0.01 mass %, andfurther still preferably free from a sulfur atom-containing frictionmodifier, based on the total amount of the lubricating oil composition.

[Various Physical Properties of Lubricating Oil Composition] <40° C.Kinematic Viscosity of Lubricating Oil Composition>

The 40° C. kinematic viscosity of the lubricating oil composition of oneaspect of the present invention is preferably 10 mm²/s to 100 mm²/s,more preferably 30 mm²/s to 60 mm²/s, further preferably 35 mm²/s to 55mm²/s, further more preferably 40 mm²/s to 50 mm²/s.

In this specification, the 40° C. kinematic viscosity of the lubricatingoil composition is a value measured according to JIS K 2283:2000.

<Oxidation Stability> (Increased Amount of Acid Value)

In the lubricating oil composition of one aspect of the presentinvention, an increased amount A acid value of the acid value before andafter a test (difference between the acid value after the test and theacid value before the test) by an oxidation stability test according toASTM D2440, which is carried out by a method described in Examplesmentioned below, is preferably lower than 0.10 mgKOH/g, more preferably0.08 mgKOH/g or less, further preferably 0.06 mgKOH/g or less, furthermore preferably 0.05 mgKOH/g or less, even further preferably 0.04mgKOH/g or less.

In this specification, the acid value is a value measured according toJIS K2501:2003-5 (Indicator titration method).

(RPVOT Value)

In the lubricating oil composition of one aspect of the presentinvention, an RPVOT value, which is measured according to ASTM D2272 ata test temperature of 150° C. after carrying out an oxidation stabilitytest according to ASTM D2440, is preferably 600 min or more, morepreferably 650 min or more, further preferably 700 min or more, furthermore preferably 750 min or more.

[Method for Producing Lubricating Oil Composition]

The production method of the lubricating oil composition of the presentinvention is not specifically limited.

For example, the method for producing the lubricating oil composition ofone aspect of the present invention is a method for producing alubricating oil composition containing a base oil (X) containingtertiary carbon atom at a content of 8.0 at % or more based on the totalcarbons of hydrocarbons, and an antioxidant (Y) containing anamine-based antioxidant (A), a phenol-based antioxidant (B) and aphosphor-based antioxidant (C),

wherein the method including a step of mixing the base oil (X) and theantioxidant (Y),

the amine-based antioxidant (A) contains a diphenylamine compound (A1)represented by the following general formula (a1):

wherein R^(a1) and R^(a2) are each independently an alkyl group having 1to 30 carbon atoms, na1 and na2 are each independently an integer of 1to 5, wherein when na1 is 2 or more, a plurality of R^(a1)'s may be thesame, or may be different from each other, and in the case where na2 is2 or more, a plurality of R^(a2)'s may be the same, or may be differentfrom each other,

the phenol-based antioxidant (B) contains a hindered phenol compound(B1) having an ester structure, and

the total content [(A1)+(B1)+(C)] of the diphenylamine compound (A1),the hindered phenol compound (B1) having an ester structure, and thephosphor-based antioxidant (C) based on the total amount of thelubricating oil composition is adjusted to be greater than 1.0 mass %and 2.0 mass % or less.

The method for mixing the above-mentioned respective components is notspecifically limited, and may include, for example, a method including astep of blending the antioxidant (Y) in the base oil (A). Theamine-based antioxidant (A), the phenol-based antioxidant (B), and thephosphor-based antioxidant (C) can be simultaneously blended to the baseoil (A), or may be blended separately. The same applies to blend ofother components (additives for a lubricating oil) than the antioxidant(Y). Incidentally, the respective components may be blended after a formof a solution (dispersion) is formed by adding a dilution oil, etc. andit is preferable to blend the respective components, and homogeneouslydisperse by stirring by a known method.

[Use of Lubricating Oil Composition]

The lubricating oil composition of one aspect of the present inventioncan be used as turbine oils for lubrication of various turbines such assteam turbines, nuclear turbines, gas turbines and hydroelectricturbines; bearing oils, gear oils, and control system hydraulic oils forlubrication of various turbomachinery such as fans and gas compressors,and further hydraulic oils.

That is, the lubricating oil composition of the present invention ispreferably used for use in lubrication of one or more devices selectedfrom the group consisting of turbines, gas compressors and hydraulicdevices.

Therefore, according to the lubricating oil composition of the presentinvention, a method for using said lubricating oil composition in one ormore devices selected from the group consisting of turbines, gascompressors and hydraulic devices is provided.

[Aspects of Present Invention Provided]

In aspects of the present invention, the following [1] to [11] areprovided.

[1] A lubricating oil composition comprising:

a base oil (X) comprising tertiary carbon atom at a content of 8.0 at %or more based on the total carbon of hydrocarbons; and

an antioxidant (Y) comprising an amine-based antioxidant (A), aphenol-based antioxidant (B), and a phosphor-based antioxidant (C),

wherein the amine-based antioxidant (A) comprises a diphenylaminecompound (A1) represented by the following general formula (a1):

wherein R^(a1) and R^(a2) are each independently an alkyl group having 1to 30 carbon atoms, na1 and na2 are each independently an integer of 1to 5, wherein when na1 is 2 or more, a plurality of R^(a1)'s may be thesame, or may be different from each other, and in the case where na2 is2 or more, a plurality of R^(a2)'s may be the same, or may be differentfrom each other,

the phenol-based antioxidant (B) comprises a hindered phenol compound(B1) having an ester structure, and

the total content [(A1)+(B1)+(C)] of the diphenylamine compound (A1),the hindered phenol compound (B1) having an ester structure, and thephosphor-based antioxidant (C) based on the total amount of thelubricating oil composition is greater than 1.0 mass % and 2.0 mass % orless.

[2] The lubricating oil composition of [1], wherein the total content[(A1)+(B1)] of the diphenylamine compound (A1) and the hindered phenolcompound (B1) having an ester structure is 0.9 mass % or more based onthe total amount of the lubricating oil composition.

[3] The lubricating oil composition of [1] or [2], wherein the contentof the diphenylamine compound (A1) is 0.30 mass % to 1.8 mass % based onthe total amount of the lubricating oil composition.

[4] The lubricating oil composition of any one of [1] to [3], whereinthe amine-based antioxidant (A) further comprises a naphthylaminecompound (A2) represented by the following general formula (a2):

wherein R^(a3) is an alkyl group having 1 to 30 carbon atoms, na3 is aninteger of 1 to 5, and when na3 is 2 or more, a plurality of R^(a3)'smay be the same or may be different from each other.

[5] The lubricating oil composition of [4], wherein the total content[(A1)+(A2)] of the diphenylamine (A1) and the naphthylamine (A2) is 0.8mass % or more and lower than 2.0 mass % based on the total amount ofthe lubricating oil composition.

[6] The lubricating oil composition of any one of [1] to [5], whereinthe hindered phenol compound (B1) having an ester structure is one ormore kind selected from the group consisting of a compound (B1-1)represented by the following general formula (b1-1) and a compound(B1-2) represented by the following general formula (b1-2):

wherein R^(b1) is an alkylene group having 1 to 5 carbon atoms, andR^(b2) is an alkyl group having 1 to 25 carbon atoms:

wherein R^(b3) and R^(b4) are each independently an alkylene grouphaving 1 to 5 carbon atoms, and m is an integer of 2 to 4.

[7] The lubricating oil composition of any one of [1] to [6], whereinthe phosphor-based antioxidant (C) comprises a phosphor atom-containingcompound (C1) having a phenol structure represented by the followinggeneral formula (c1).

wherein R^(c1), R^(c2), R^(c3) and R^(c4) are each independently ahydrogen atom or an alkyl group having 1 to 30 carbon atoms.

[8] The lubricating oil composition of any one of [1] to [7], furthercomprising one or more kind of lubricating oil additive selected fromthe group consisting of an extreme pressure agent, a detergentdispersant, a pour point depressant, a viscosity index improver, a rustinhibitor, a metal deactivator, an anti-foaming agent and a frictionmodifier.

[9] The lubricating oil composition of any one of [1] to [8], whereinthe content of the base oil (X) is 90.0 mass % or more based on thetotal amount of the lubricating oil composition.

[10] The lubricating oil composition of any one of [1] to [9], which isused for one or more device selected from the group consisting of aturbine, a compressor and a hydraulic device.

[11] A method for using the lubricating oil composition of any one of[1] to [9] in one or more device selected from the group consisting of aturbine, a compressor and a hydraulic device.

EXAMPLES

The present invention will be described more specifically with referenceto examples, but the present invention is not limited to these examples.

[Method for Measuring Various Physical Properties]

The respective characteristics of the respective raw material and thelubricating oil compositions used in the respective Examples andrespective Comparative Examples are measured according to the proceduresshown below.

(1) Kinematic Viscosity and Viscosity Index

The kinematic viscosity and the viscosity index were measured andcalculated in accordance with JIS K2283:2000.

(2) Acid Value

The acid value was measured in accordance with JIS K2501:2003-5(Indicator titration method).

(3) Tertiary Carbon Atom Content

The tertiary carbon atom content (based on the total carbons ofhydrocarbons) of the base oil was measured by the above-mentioned methodusing ¹³C-NMR.

Examples 1 to 3 and Comparative Examples 1 to 8

The base oil and various additives shown below were sufficiently mixedat the contents (mass %) shown in Table 1 to respectively preparelubricating oil compositions.

The details of the base oils and various additives used in Examples 1 to3 and Comparative Examples 1 to 8 are as shown below.

<Base Oil (X)>

Base Oil (X1)

A base oil belonging to Group III by API Classification, which isobtained by subjecting a raw material oil containing a petroleum-derivedwax to a hydrogenation isomerization dewaxing treatment, then subjectingto a hydrogenation finishing treatment.

Tertiary carbon atom content: 10.0 at % (base oil based on the totalamount), 40° C. kinematic viscosity: 43.75 mm²/s, viscosity index: 143

Base Oil (X2)

Base Oil Containing Isoparaffin Polymer

Tertiary carbon atom content: 9.2 at % (based on the total carbons ofhydrocarbons in base oil), 40° C. kinematic viscosity: 45.87 mm²/s,viscosity index: 107

Base Oil (X3)

Mixed base oil having a tertiary carbon atom content of 5.0 at % (baseoil based on the total amount) obtained by mixing two kinds of thefollowing (1) and (2)

(1) A base oil belonging to Group II by API classification, tertiarycarbon atom content: 5.0 at % (based on the total carbons ofhydrocarbons in base oil), 40° C. kinematic viscosity: 30.60 mm²/s,viscosity index: 104

(2) A base oil belonging to Group II by API classification, tertiarycarbon atom content: 5.0 at % (based on the total carbons ofhydrocarbons in base oil), 40° C. kinematic viscosity: 90.51 mm²/s,viscosity index: 107

-   -   The content ratio of (1) to (2) is (1):(2)=67.90:30.00 by mass        ratio.

Base Oil (X4)

Base Oil Formed of Poly-α-Olefin

Tertiary carbon atom content: 5.0 at % (based on the total carbons ofhydrocarbons in base oil), 40° C. kinematic viscosity: 46.74 mm²/s,viscosity index: 137

<Amine-Based Antioxidant (A)>

Diphenylamine (A1): Di(Octylphenyl)Amine

A compound wherein, in the general formula (a1), R^(a1) and R^(a2) areoctyl groups, and na1=na2=1.

Naphthylamine (A2): Octylphenyl-α-Naphthylamine

A compound wherein, in the general formula (a2), R^(a3) is an octylgroup and na3=1.

<Phenol-Based Antioxidant (B)>

Hindered Phenol Compound (B1): BenzenepropanoicAcid-3,5-Bis(1,1-Dimethylethyl)-4-Hydroxyalkyl Ester

A compound wherein, in the general formula (b1-1), R^(b1) is an alkylenegroup having two carbon atoms, and R^(b2) is an alkyl group having 8carbon atoms.

<Phosphor-Based Antioxidant (C)>

Phosphor Atom-Containing Compound (C1) Having a Phenol Structure:Diethyl Di-Tert-Butyl-4-Hydroxybenzylphosphate

A compound wherein, in the general formula (c1), R^(c1) and R^(c2) aretert-butyl groups, and R^(c3) and R^(c4) are ethyl groups.

<Additives for Lubricating Oil>

-   -   Extreme pressure agent: dithiophosphoric acid    -   Rust inhibitor: an alkenyl succinate polyhydric alcohol ester,    -   Metal deactivator: an N-dialkylaminomethylbenzotriazole    -   Anti-foaming agent: a silicone-based anti-foaming agent having a        resin content concentration of 1 mass %

For each of the lubricating oil compositions of Examples 1 to 3 andComparative Examples 1 to 8, the following two tests were carried out.

<Oxidation Stability Test 1 (Evaluation of Increased Amount of AcidValue Before and After Test)>

First, the acid value before test of each lubricating oil compositionwas measured. Second, an oxidation stability test was carried out underthe following conditions in accordance with ASTM D2440. In this test,raising of an acid value at the initial stage was confirmed with settingthe test time to a week (168 hours).

-   -   Test temperature: 110° C.    -   Catalyst: copper    -   Test time: 1 week (168 hours)    -   Oxygen blow amount: 1 L/h

After the test, the acid value of each lubricating oil composition wasmeasured, and the amount of raising from the acid value before the test(A acid value) was obtained. Furthermore, the lubricating oilcomposition having A acid value of lower than 0.10 mgKOH/g was judged tobe acceptable.

<Oxidation Stability Test 2 (Evaluation of RPVOT Values Before and AfterTest)>

First, the RPVOT value of each lubricating oil composition was measuredin accordance with ASTM D2272 at a test temperature of 150° C. Second,the RPVOT value of each lubricating oil composition after the oxidationstability test was carried out was similarly measured in accordance withASTM D2440. In this test, the residual lifetime of the test oil wasconfirmed from the decrease in performance of the antioxidant.

Furthermore, the test oil having a RPVOT value of 600 min or more wasjudged to be acceptable.

The results are shown in Table 1.

TABLE 1 Examples Comparative Examples Unit 1 2 3 1 2 3 Lubricating oilBase oil Base oil (X1) mass % 97.90 97.40 — 96.60 96.60 98.40composition (X) Base oil (X2) mass % — — 97.90 — — — Base oil (X3) mass% — — — — — — Base oil (X4) mass % — — — — — — Antioxidant Amine-basedDiphenylamine mass % 0.50 1.00 0.50 2.00 1.00 — (Y) antioxidant (A) (A1)Naphthylamine mass % 0.60 0.60 0.60 — 1.00 0.60 (A2) Phenol-basedHindered phenol mass % 0.60 0.60 0.60 1.00 1.00 0.60 antioxidant (B)compound (B1) having an ester structure Phosphor-based Phosphor mass %0.10 0.10 0.10 0.10 0.10 0.10 antioxidant (C) atom-containing compound(C1) having phenol structure Additives for Pour point depressant mass %0.05 0.05 0.05 0.05 0.05 0.05 lubricating oil Rust inhibitor mass % 0.050.05 0.05 0.05 0.05 0.05 Metal deactivator mass % 0.10 0.10 0.10 0.100.10 0.10 Anti-foaming agent mass % 0.10 0.10 0.10 0.10 0.10 0.10 Totalmass % 100.00 100.00 100.00 100.00 100.00 100.00 Tertiary carbon atomcontent of base oil (X) at % 10.0 10.0 9.2 10.0 10.0 10.0 (A1) + (B1) +(C) mass % 1.2 1.7 1.2 3.1 2.1 0.7 (A1) + (B1) mass % 1.1 1.6 1.1 3.02.0 0.6 (A1)/(B1) mass % 0.8 1.7 0.8 2.0 1.0 0.0 (A1)/(C) mass % 5.010.0 5.0 20.0 10.0 0.0 (B1)/(C) mass % 6.0 6.0 6.0 10.0 10.0 6.0 (A1) +(A2) mass % 1.10 1.60 1.10 2.00 2.00 0.60 Evaluation Fresh oil Acidvalue mgKOH/g 0.26 0.27 0.25 0.27 0.25 0.28 RPVOT value min 1633 17771555 1769 1656 796 Test oil Acid value mgKOH/g 0.24 0.27 0.28 0.52 0.350.54 RPVOT value min 865 990 789 851 821 330 ΔAcid value mgKOH/g 0.020.00 0.03 0.25 0.10 0.26 Comparative Examples Unit 4 5 6 7 8 Lubricatingoil Base oil Base oil (X1) mass % 97.60 98.50 — — 98.00 composition (X)Base oil (X2) mass % — — — — — Base oil (X3) mass % — — 97.90 — — Baseoil (X4) mass % — — — 97.90 — Antioxidant Amine-based Diphenylamine mass% 1.00 0.50 0.50 0.50 0.50 (Y) antioxidant (A) (A1) Naphthylamine mass %— 0.60 0.60 0.60 0.60 (A2) Phenol-based Hindered phenol mass % 1.00 —0.60 0.60 0.60 antioxidant (B) compound (B1) having an ester structurePhosphor-based Phosphor mass % 0.10 0.10 0.10 0.10 — antioxidant (C)atom-containing compound (C1) having phenol structure Additives for Pourpoint depressant mass % 0.05 0.05 0.05 0.05 0.05 lubricating oil Rustinhibitor mass % 0.05 0.05 0.05 0.05 0.05 Metal deactivator mass % 0.100.10 0.10 0.10 0.10 Anti-foaming agent mass % 0.10 0.10 0.10 0.10 0.10Total mass % 100.00 100.00 100.00 100.00 100.00 Tertiary carbon atomcontent of base oil (X) at % 10.0 10.0 5.0 5.0 10.0 (A1) + (B1) + (C)mass % 2.1 0.6 1.2 1.2 1.0 (A1) + (B1) mass % 2.0 0.5 1.1 1.1 1.1(A1)/(B1) mass % 1.0 — 0.8 0.8 0.8 (A1)/(C) mass % 10.0 5.0 5.0 5.0 —(B1)/(C) mass % 10.0 0.0 6.0 6.0 — (A1) + (A2) mass % 1.00 1.10 1.101.10 1.10 Evaluation Fresh oil Acid value mgKOH/g 0.28 0.29 0.23 0.260.24 RPVOT value min 1372 3046 1090 1180 1565 Test oil Acid valuemgKOH/g 0.41 0.24 0.26 0.26 0.31 RPVOT value min 1023 232 456 542 348ΔAcid value mgKOH/g 0.13 0.05 0.03 0.00 0.07

The following matters are understood from Table 1.

It is understood that the lubricating oil compositions of Examples 1 to3 gave excellent results for both Δacid value and RPVOT value, and thusare lubricating oil compositions capable of suppressing raising of anacid value at the initial stage and maintaining oxidation stability fora long period of time.

On the other hand, it is understood that, in the lubricating oilcompositions in which the total content [(A1)+(B1)+(C)] of thediphenylamine compound (A1), the hindered phenol compound (B1) and thephosphor-based antioxidant (C) is greater than 2.0 mass % likeComparative Examples 1, 2 and 4, the value of A acid value is large, andraising of an acid value at the initial stage occurs easily.

Furthermore, it is understood that, in the lubricating oil compositionsin which the total content [(A1)+(B1)+(C)] of the diphenylamine compound(A1), the hindered phenol compound (B1), and the phosphor-basedantioxidant (C) is 1.0 mass % or less like Comparative Examples 3, 5 and8, the RPVOT value decreases, and thus oxidation stability cannot bemaintained for a long period of time.

Moreover, in the lubricating oil compositions using a base oil having atertiary carbon atom content of lower than 8.0 at % like ComparativeExamples 6 and 7, the value of Δacid value is small, and thus raising ofan acid value at the initial stage is difficult to occur, whereas theRPVOT value decreases, and thus oxidation stability cannot be maintainedfor a long period of time.

1. A lubricating oil composition comprising: a base oil (X) comprisingtertiary carbon atom at a content of 8.0 at % or more based on the totalcarbon of hydrocarbons; and an antioxidant (Y) comprising an amine-basedantioxidant (A), a phenol-based antioxidant (B), and a phosphor-basedantioxidant (C), wherein the amine-based antioxidant (A) comprises adiphenylamine compound (A1) represented by the following general formula(a1):

wherein R^(a1) and R^(a2) are each independently an alkyl group having 1to 30 carbon atoms, na1 and na2 are each independently an integer of 1to 5, wherein when na1 is 2 or more, a plurality of Rai's may be thesame, or may be different from each other, and in the case where na2 is2 or more, a plurality of R^(a2)'s may be the same, or may be differentfrom each other, the phenol-based antioxidant (B) comprises a hinderedphenol compound (B1) having an ester structure, and the total content[(A1)+(B1)+(C)] of the diphenylamine compound (A1), the hindered phenolcompound (B1) having an ester structure, and the phosphor-basedantioxidant (C) based on the total amount of the lubricating oilcomposition is greater than 1.0 mass % and 2.0 mass % or less.
 2. Thelubricating oil composition according to claim 1, wherein the totalcontent [(A1)+(B1)] of the diphenylamine compound (A1) and the hinderedphenol compound (B1) having an ester structure is 0.9 mass % or morebased on the total amount of the lubricating oil composition.
 3. Thelubricating oil composition according to claim 1, wherein the content ofthe diphenylamine compound (A1) is 0.3 mass % to 1.8 mass % based on thetotal amount of the lubricating oil composition.
 4. The lubricating oilcomposition according to claim 1, wherein the amine-based antioxidant(A) further comprises a naphthylamine compound (A2) represented by thefollowing general formula (a2):

wherein R^(a3) is an alkyl group having 1 to 30 carbon atoms, na3 is aninteger of 1 to 5, and when na3 is 2 or more, a plurality of R^(a3)'smay be the same or may be different from each other.
 5. The lubricatingoil composition according to claim 4, wherein the total content[(A1)+(A2)] of the diphenylamine (A1) and the naphthylamine (A2) is 0.8mass % or more and lower than 2.0 mass % based on the total amount ofthe lubricating oil composition.
 6. The lubricating oil compositionaccording to claim 1, wherein the hindered phenol compound (B1) havingan ester structure is one or more selected from the group consisting ofa compound (B1-1) represented by the following general formula (b1-1)and a compound (B1-2) represented by the following general formula(b1-2):

wherein R^(b1) is an alkylene group having 1 to 5 carbon atoms, andR^(b2) is an alkyl group having 1 to 25 carbon atoms;

wherein R^(b3) and R^(b4) are each independently an alkylene grouphaving 1 to 5 carbon atoms, and m is an integer of 2 to
 4. 7. Thelubricating oil composition according to claim 1, wherein thephosphor-based antioxidant (C) comprises a phosphor atom-containingcompound (C1) having a phenol structure represented by the followinggeneral formula (c1):

wherein R^(c1), R^(c2), R^(c3) and R^(c4) are each independently ahydrogen atom or an alkyl group having 1 to 30 carbon atoms.
 8. Thelubricating oil composition according to claim 1, further comprising oneor more of lubricating oil additive selected from the group consistingof an extreme pressure agent, a detergent dispersant, a pour pointdepressant, a viscosity index improver, a rust inhibitor, a metaldeactivator, an anti-foaming agent and a friction modifier.
 9. Thelubricating oil composition according to claim 1, wherein the content ofthe base oil (X) is 90.0 mass % or more based on the total amount of thelubricating oil composition.
 10. The lubricating oil compositionaccording to claim 1, which is used for one or more device selected fromthe group consisting of a turbine, a compressor and a hydraulic device.11. A method for using the lubricating oil composition according toclaim 1, in one or more device selected from the group consisting of aturbine, a compressor and a hydraulic device.